This Program Project renewal application continues to have a narrow focus on the role of enterocytes in mucosal barrier function at the interface between microbial and enterotoxin-mediated stimuli and immune effector responses. The enterocyte is the central focus and will be studied with regard to microbial "crosstalk" and immune-epithelial cell interactions, neuropeptide receptor expression, inappropriate developmental responses, and a barrier to microbial penetration. This renewal application consists of five interactive projects supported by two cores, (1) An Administrative and (2) Xenograft/lsograft Transplant, human intestinal tissue and imaging core principally in one location in the Mucosal Immunology Laboratory at the Massachusetts General Hospital-East (BIdg 114). Project 1 will examine immaturities in NFkappaB/MyD88 innate response genes and in other inflammatory pathways in fetal vs. mature enterocytes to help explain excessive intestinal inflammation in prematures and test maturational prevention with hydrocortisone and probiotics. Project 2 will examine the mechanisms involved in the participation of corticotrophin releasing hormone family of neuropeptides and their receptors in the development of mucosal inflammation in response to enterotoxins and bacterial pathogens. Project 3 will study the molecular mechanisms underlying S. flexneri-intestinal epithelial interactions at the apical or basolateral membrane domain that lead to acute infectious colitis. Project 4 will determine the role of GEF-H1, a guanine nucleotide exchange factor (GEF) for Rho, on epithelial cell responses to pathogens and examine the mechanism of this response at the tight junctional level. Project 5 will define specific cellular and molecular mechanisms involved in the protective and therapeutic effects of the probiotic agent .S.boulardii in enteric infections and enterotoxin-mediated diarrhea and intestinal inflammation. Investigators with disciplines in cell/molecular biology, microbiology, intestinal immunity and inflammation and developmental biology will provide a better understanding of the pathogenesis of bacterial Gl infections and enterotoxin-mediated intestinal inflammation and may lead to new therapeutic strategies in preventing and treating infectious diseases in the Gl tract. PUBLIC HEALTH RELEVANCE: This Program Project examines how colonizing bacteria stimulate intestinal host defense and prevents inflammation by pathogens and their toxins as a translation contribution. Observations made may lead to strategies for prevention of intestinal infectious disease. TABLE OF CONTENTS Overall Description 2